1. Field of the Invention
The present invention relates to a zoom lens system preferable for use in a digital still camera, a video camera or the like.
2. Description of Related Art
A known zoom lens which has a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power in order from an object side achieves zooming by moving the first, second, third, and fourth lens units (for example, see Patent Documents 1 to 3).
Another known zoom lens has a four lens unit structure as described above including a third lens unit formed to have a plurality of lenses disposed with a large air spacing (for example, see Patent Documents 4, 5).
In addition, a known zoom lens of a rear focus type achieves focusing by moving lens units other than a first lens unit on an object side as a means for realizing reductions in overall length of the zoom lens and a front element diameter (Patent Documents 6, 7).
Generally, in the zoom lens of the rear focus type, the effective diameter of a first lens unit can be reduced as compared with a zoom lens which achieves focusing by moving a first lens unit, so that the size of the overall lens system can be easily reduced. Also, close-up, especially image-taking at the closest focusing distance is easily performed. Since small and lightweight lens units are moved, only a small driving force is needed for the lens units to allow fast focusing.
(Patent Document 1)
Japanese Patent Application Laid-Open No. H08(1996)-50244
(Patent Document 2)
U.S. Pat. No. 4,632,519
(Patent Document 3)
Japanese Patent Application Laid-Open No. 2001-194586 (corresponding to U.S. Pat. No. 6,456,441)
(Patent Document 4)
Japanese Patent Application Laid-Open No. 2001-242379
(Patent Document 5)
Japanese Patent Application Laid-Open No. 2001-356269
(Patent Document 6)
Japanese Patent Application Laid-Open No. H11(1999)-305124 (corresponding to U.S. Pat. No. 6,166,864)
(Patent Document 7)
Japanese Patent Application Laid-Open No. H10(1998)-62687 (corresponding to U.S. Pat. No. 6,016,228)
In a zoom lens disclosed in Japanese Patent Application Laid-Open No. H08(1996)-50244, a third lens unit is formed of a single positive lens in which the effect of variable magnification is partially provided by moving the third lens unit. When a high zoom ratio is provided, variations in aberration of the third lens present a problem.
In U.S. Pat. No. 4,632,519, the zoom ratio is approximately 5.7. The spacing between a third lens unit and a fourth lens unit becomes smaller from the wide-angle end to the telephoto end to reduce the action of variable magnification provided by the third lens unit. Especially for a high zoom ratio of 6 or more, the overall length of the zoom lens is increased at the telephoto end to present a problem in terms of a reduction in size.
In Japanese Patent Application Laid-Open Nos. 2001-242379 and 2001-356269, the variable magnification ratio is approximately 3, and a first lens unit is formed of a single positive lens. Thus, when a higher zoom ratio is provided, variations in aberration occurring in the first lens unit during variation of magnification are not easily canceled by the other lens units.
On the other hand, in a camera such as a video camera and a digital still camera, a solid-state image-pickup device with a number of pixels (a multi-pixel image-pickup device) is often used. A high-performance zoom lens is required as an optical system for use in such a camera.
It is necessary, especially for a zoom lens for a multi-pixel image-pickup device, not only to correct monochromatic aberration but also to sufficiently correct chromatic aberration in a wide wavelength range. Generally, when a zoom lens with a high zoom ratio has a large focal length of the overall system at a zoom position on the telephoto side, a reduced secondary spectrum is highly demanded in chromatic aberration in addition to primary achromatism.
Conventionally, a number of zoom lenses have been known which employ a lens made of glass with anomalous dispersion to correct a secondary spectrum of axial chromatic aberration (longitudinal chromatic aberration) at a zoom position on the telephoto side. In addition, as a structure of a zoom lens suitable for providing a high zoom ratio, a positive lead type zoom lens is an example which includes a lens unit having a positive refractive power closest to an object.
A known zoom lens has a three lens unit structure with a positive, a negative, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 8 to 10).
Another known zoom lens has a four lens unit structure with a positive, a negative, a positive, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 11 to 15).
Another known zoom lens has a five lens unit structure with a positive, a negative, a positive, a negative, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 12, 15 to 17).
(Patent Document 8)
Japanese Patent No. 3008580 (corresponding to U.S. Pat. No. 5,257,134)
(Patent Document 9)
Japanese Patent Application Laid-Open No. H06(1994)-43363
(Patent Document 10)
Japanese Patent Publication No. H03(1991)-58490 (corresponding to U.S. Pat. No. 4,709,997)
(Patent Document 11)
Japanese Patent No. 3097399
(Patent Document 12)
Japanese Patent Application Laid-Open No. 2002-62478 (corresponding to U.S. Pat. No. 6,594,087)
(Patent Document 13)
Japanese Patent Application Laid-Open No. 2000-321499 (corresponding to U.S. Pat. No. 6,414,799)
(Patent Document 14)
Japanese Patent Application Laid-Open No. H08(1996)-248317
(Patent Document 15)
Japanese Patent Application Laid-Open No. 2001-194590 (corresponding to U.S. Pat. No. 6,404,561)
(Patent Document 16)
Japanese Patent Application Laid-Open No. H09(1997)-5624 (corresponding to U.S. Pat. No. 5,760,966)
(Patent Document 17)
Japanese Patent Application Laid-Open No. 2001-350093 (corresponding to U.S. Pat. No. 6,449,433)
In the positive lead type zoom lens, the secondary spectrum of axial chromatic aberration at a zoom position on the telephoto side is likely to occur in the first lens unit having a positive refractive power in which the height of axial rays is high. Thus, the positive lens of the first lens unit is often made of glass with anomalous dispersion to reduce the secondary spectrum. However, the glass with anomalous dispersion is typically difficult to process as compared with normal glass, and especially when it is used for the first lens unit having a large effective diameter, it is difficult to provide a lens with high processing accuracy.
In a zoom lens formed of three lens units having a positive, a negative, and a positive refractive powers in order from an object side, the height of axial rays is high in the third lens unit, so that a significant effect of correcting the secondary spectrum of axial chromatic aberration is achieved by using glass with anomalous dispersion for a material of a positive lens of the third lens unit. This is advantageous in manufacture since the third lens unit has a smaller lens effective diameter than a first lens unit.
In Patent Documents 14, 15, and 17, a first lens unit having a positive refractive power includes a lens made of glass with anomalous dispersion. However, a third lens unit has no lens made of glass with anomalous dispersion, and correction of chromatic aberration is not necessarily sufficient.
In Patent Document 13, a fourth lens unit has a lens made of glass with anomalous dispersion, but any of a first lens unit and a third lens unit has no lens made of glass with anomalous dispersion. The lens made of the glass with anomalous dispersion in the fourth lens unit effectively reduces the secondary spectrum of chromatic aberration of magnification (lateral chromatic aberration). The zoom ratio in an embodiment is approximately 4, but when the zoom ratio is increased, correction of the axial secondary spectrum is not necessarily sufficient on the telephoto side.
Generally, in a zoom lens having a three lens unit structure with a positive, a negative, and a positive refractive powers in order from an object side or having a four lens unit structure, the lens arrangement in which a first lens unit is moved during variation of magnification is suitable for reducing the overall length of the zoom lens at the wide-angle end at a high zoom ratio of 7 or more.
In Patent Documents 8 to 11 and 16 described above, however, the first lens unit is fixed during variation of magnification. Thus, it is difficult to achieve both of a smaller overall length of the zoom lens and high variable magnification.
In Patent Documents 10 and 11, each of a first lens unit and a third lens unit employs a lens made of lens with anomalous dispersion to favorably correct chromatic aberration. However, the first lens unit employs the lens, so that the lens tends to have a large effective diameter to cause difficulty in manufacturing the lens.
In Patent Document 11, since an aperture stop is provided in the fourth lens unit, the diameter of a front element is increased when the focal length at the wide-angle end is reduced to obtain a wider angle. In each embodiment of Patent Document 11, half of the field angle at the wide-angle end is as narrow as 16.7 degrees to make the lens inappropriate for specifications including half of the field angle larger than 30 degrees.
Generally, the use of a number of lenses made of a glass material with high anomalous dispersion is effective in enhancing the effect of correcting the secondary spectrum. A particularly effective glass material has an Abbe number larger than 90 and a partial dispersion ratio Θg, F lager than 0.53 (for example, fluorite). In any of the conventional examples, such a glass material with high anomalous dispersion is not used. Thus, the correction of the secondary spectrum is not sufficient when the number of pixels is increased to cause a smaller pitch in a solid-state image-pickup device in a digital camera or the like.